The following discussion is not an admission that anything discussed below is common general knowledge or citable as prior art.
Anaerobic digestion produces biogas as a result of the biological fermentation of volatile solids (VS) supplied with the feedstock. The degree of volatile solids reduction is related to the biodegradability of the feedstock and process conditions in the digester. Important digester parameters or considerations include temperature, pH, food-to-microorganism ratio, organic loading rate, hydraulic and solids retention time, absence of toxic substances at inhibitory concentrations, adequate mixing, and others. The closer these conditions are to an optimum, the higher the VS reduction will be.
Typically, digesters treating complex organic substrates will achieve 60 to 80% VS reduction. With substrates with high fiber content, such as silage or dairy manure, a digester may achieve about 60% VS destruction. Typical digesters treating municipal sewage sludge produced in a wastewater treatment plant usually achieve about 50% VS destruction in 20 day hydraulic retention time (HRT) mesophilic digesters.
The digester sludge, or digestate, produced by an anaerobic digester is a combination of inert solids that were fed with the substrate, recalcitrant volatile solids that could not be degraded biologically, and bacterial biomass that grew as a result of feeding on the degradable portion of the volatile solids fed with the feedstock. A typical digestate solids content is 2% to 10% total solids (TS) or dried solids (DS), depending on the substrate and the type of digester. The digestate may be dewatered mechanically to produce a cake with 20 to 30% solids, depending among other things on the undigested fiber content and the type of dewatering device used.
Pyrolysis is a technique typically used to process solid waste such as wood chips or sawdust. Pyrolysis produces biochar, liquids and gases from a biomass by heating the biomass in a low or no oxygen environment. The absence or deficiency of oxygen prevents combustion. The relative yield of products from pyrolysis varies with temperature. Temperatures of 400-500° C. (752-932° F.) produce more char, while higher temperatures, up to and above 700° C. (1,292° F.) favor the yield of liquid and gas fuel components. Pyrolysis occurs more quickly at the higher temperatures, typically requiring seconds instead of hours. High temperature pyrolysis is also known as gasification, and produces primarily synthesis gas. Once initiated, both processes can be self supporting and produce net energy, not accounting for the energy value of the biomass consumed.
Synthesis gas, also called syngas or producer gas, is a combination of CO, H2 and CO2 that results from the thermal degradation of biomass without combustion, through pyrolysis or gasification. This process occurs typically at temperatures between 500 and 700 deg C. with minimal introduction of oxygen, if any. Other components of syngas are water vapor, methane, light hydrocarbons, particulates and volatile impurities. Syngas has a low heat value, for example 120 to 150 Btu/cubic foot. After adequate cleaning to remove moisture, particulates, tars and impurities, syngas can be used for heat or electricity production with adapted internal combustion engines, boilers, gas turbines, or fuel cells. In some gasification systems, syngas is subject to high temperature steam reformation to produce hydrogen that is either sold as a gaseous fuel or used on site to power fuel cells.
In some cases, syngas is upgraded to methane with chemical catalysts. In one process, a water-gas shift (WGS) step increases the H2/CO ratio of the syngas and is followed by a nickel catalyst supported step that enables conversion from CO and CO2 into methane and water. This process is costly due to the chemical catalyst, its energy demand, and the need to pre-treat the syngas to remove impurities.